The long-term objective of this proposal is to determine the tumor suppressor functions of the hereditary breast/ovarian cancer susceptibility gene, BRCA1. BRCA1 and its associated proteins are known to contribute to homologous recombination (HR), a potentially error-free form of double strand break repair. However, the precise steps in HR that are regulated by BRCA1 are not well understood. Nor is it understood to what extent BRCA1's role in HR accounts for its function as a tumor suppressor gene. We propose that BRCA1 acts as a tumor suppressor by controlling sister chromatid recombination (SCR), an HR process that operates during S and G2 phases of the cell cycle to repair replication-associated DNA damage in an error free manner. To test this hypothesis, we have developed novel SCR reporters that deliver an unprecedented degree of detailed molecular information regarding SCR, and will use them to study BRCA1's role in this process. Our data suggest that cells lacking wild-type BRCA1 have a specific defect in SCR. In work proposed here, we will explore this observation in depth and attempt to relate it to BRCA1's function as a tumor suppressor. Our specific aims are: 1. To define SCR functions of BRCA1. 2. To determine whether clinically described mutant BRCA1 alleles are defective for SCR control. 3. To identify steps during DSB processing and SCR that are regulated by BRCA1.